Evolution Stones (commonly known as Elemental Stones) are crystals with the power of a certain element that instantly evolve certain Pokémon they are used on. There are currently nine different stones for evolving Pokémon.
Fire, Water, and Thunder Stones evolve Pokémon of the corresponding Pokémon types, for example Vulpix, Staryu, and Pikachu, respectively.
There are a handful of other stones related to evolution.
The Everstone prevents the Pokémon that holds it from evolving.
The Sun Shard and Moon Shard (found in Pokémon XD only) simulate the effects of Day and Night, which allows a happy Eevee to evolve into Espeon or Umbreon.
The Thunder Stone is a special elemental stone, introduced in Generation I, which when used on certain Pokémon makes them evolve. It is an elliptical-shaped stone that is green in appearance, with a thunderbolt insignia inside of it, and evolves Eevee into Jolteon, Pikachu into Raichu, and Eelektrik into Eelektross.
The Fire Stone is a special elemental stone, introduced in Generation I, which when used on certain Pokémon makes them evolve. It is an elliptical-shaped stone that is yellow in appearance, with a fire insignia inside of it, and usually makes Fire-type Pokémon evolve. It evolves Eevee into Flareon, Vulpix into Ninetales, Growlithe into Arcanine, and Pansear into Simisear.
The Moon Stone is a special elemental stone, introduced in Generation I, which when used on certain Pokémon makes them evolve. It is an elliptical-shaped stone that is grey in appearance, looks somewhat like a moon rock, and usually makes Fairy-Group Pokémon evolve. It evolves Jigglypuff into Wigglytuff, Skitty into Delcatty,Clefairy into Clefable, Nidorino into Nidoking, Nidorina into Nidoqueen, and Munna into Musharna.
The Sun Stone is a special stone, introduced in Generation II, which when used on certain Pokémon makes them evolve. It is orange in appearance, in the shape of the sun, and usually makes plant Pokémon with some connection to the sun evolve. It evolves Sunkern into Sunflora, Gloom into Bellossom, Petilil into Lilligant, and Cottenee into Whimsicott.
The Dawn Stone is a peculiar stone, introduced in Generation IV, that will only evolve Pokémon of a determinate gender. It is a round stone, sky blue in appearance, and actually looks similar to an opal. It evolves a female Snorunt into Froslass and a male Kirlia into Gallade.
A Dusk Stone is a very odd stone which was introduced in Generation IV, that will evolve certain Pokémon that are associated with nighttime. It is a purplish-black, irregularly-shaped stone.
The Shiny Stone is a special elemental stone, introduced in Generation IV, which when used on only certain Fairy-Group Pokémon, makes them evolve. It is an elipitcal-shaped stone that is transparent in appearance, with a ball of light inside of it. It evolves Togetic into Togekiss, Roselia into Roserade, and Minccino into Cinccino.